The present invention relates to acceleration enrichment for petrol injection systems.
Petrol consists of chains of hydrocarbons of varying length. As temperature increases and pressure decreases, even the longer molecule chains vaporize.
during idling conditions in petrol injection systems, a vacuum is present in the inlet manifold downstream of the throttle valve. The injected petrol vaporize completely and passes into the cylinder. However, as the throttle valve is opened, the intake mainfold pressure increases correspondingly. The tendency of the fuel to vaporize then decreases, the result being that longer fuel molecule chains are deposited in liquid form as a film on the wall of the intake manifold. The latter quantity of fuel is not combusted and the mixture which is actually combusted is too lean. The acceptance of petrol is thus poor during acceleration conditions. It is the object of acceleration enrichment (BA) to provide an excess quantity of fuel during acceleration so that the engine receives the correct mixture composition during acceleration despite the formation of the film on the wall.
This excess quantity is determined during initial installation of new engines and is stored permanently in the data store of the control device of the fuel injection system.
It has recently been established, however, that coking of the inlet valves occurs following a longish operating time and dependent upon the type of petrol used and the driver's driving technique. This has a deleterious effect on acceleration, since the coking on the intake valve acts during acceleration as a sponge in addition to the film on the wall. Fuel drops are caught in the coked, porous surface of the intake valve and are not combusted. As a consequence of the resulting too-lean mixture, the engine torque drops considerably. In the worst cases, the engine can actually stop during an acceleration demand. If the acceleration enrichment quantity is increased considerably, normal driving is once again possible. However, this excess quantity cannot be provided for in a new engine, since it would not then be possible to adhere to legal exhaust-gas limitations. Also, the driving performance of new vehicles would be poorer, because over-enrichment would cause the engine torque to drop during acceleration. A method is therefore required which automatically adapts the excess acceleration quantity to engine conditions.
Some adaptive methods for acceleration enrichment are already known, e.g. as described in DE-OS 2 841 268 (GB-PS No. 20 30 730) and US-PS No. 4 245 312.
However, these known methods use only the information from a conventional lambda (air-fuel ratio .lambda.) regulator for the adaptation. Conventional lambda regulators are, however, only activated at engine temperatures of above 20.degree. C. Below this temperature, there is controlled driving only, because an engine requires a richer mixture than lambda=1. In addition, there are no legal exhaust-gas regulations effective below this temperature. The only criterion in this range is the driving performance. Up till now, the only technique available has been to apply to cold engines adaption values established for a warm engine, without the accuracy thereof being tested.
It has now been determined using some actual examples of coked intake valves that the acceleration enrichment factor for a warm engine must be increased some five-fold with respect to the new state in order for lambda=1 to be obtained again during acceleration enrichment. In the known methods, in the case of a cold engine (-30 degrees . . . +20 degrees), the acceleration enrichment, which has been considerably increased over that for a warm engine, is increased by a further factor of 0.5 during engine warm-up. There is thus a risk of over-enrichment.
It is an object of the present invention to provide a technique of adaptive acceleration enrichment which overcomes the above-discussed problems of the known solutions.